The present invention is based on a fuel injector according to the definition of the species of Claim 1 and a method of operating a fuel injector according to the definition of the species of Claim 7.
German Patent Application 195 00 706 A1 describes a fuel injector of the type defined in the main claim. In the fuel injector described in this document, a piezoelectric actuator is provided for actuating a valve needle connected to a valve closing body. The valve closing body works together with a valve seat surface to form a sealing seat. The fuel injector can be designed as either an outward-opening or an inward-opening fuel injector. The piezoelectric actuator formed by a plurality of stacked piezoelectric layers generates relatively high displacement forces, but relatively short displacement paths. Therefore, the known document proposes that a hydraulic step-up mechanism be provided to magnify the displacement path between the valve needle and the piezoelectric actuator and transmitted to the valve needle. The hydraulic step-up mechanism provides temperature compensation of the piezoelectric actuator at the same time.
As known, the piezoelectric actuator is subject to non-negligible temperature-dependent elongation. This temperature-dependent elongation of the piezoelectric actuator is, however, relatively slow compared to the actuator""s actuating stroke which results in the opening of the fuel injector. Therefore, the temperature-dependent elongation of the actuator is a quasi-static process. The associated displacement of the hydraulic medium does not result in opening of the fuel injector, but the displaced hydraulic medium is vented quasi-statically via the guide gaps of the hydraulic step-up mechanism.
In some applications the operating stroke of the actuator does not have to be hydraulically stepped up, since the actuator generates sufficient stroke for opening the fuel injector. For such applications, the arrangement of a hydraulic step-up mechanism for the purpose of temperature compensation only would be too cost-intensive and complicated. Furthermore, it is disadvantageous that a special hydraulic medium must be used for the hydraulic step-up mechanism, which may leak out over time. This may negatively affect the operation of the step-up mechanism and the service life of the fuel injector.
German Patent 43 06 073 C1 describes a fuel injector having a piezoelectric actuator in a different design. Also in this fuel injector, temperature is compensated via a hydraulic step-up mechanism. German Patent Application 35 33 085 A1 describes a fuel injector without a hydraulic step-up mechanism, but also without any temperature compensation.
The fuel injector according to the present invention having the features of Claim 1 has the advantage over the related art that the piezoelectric or magnetostrictive actuator is temperature compensated due to the gap arranged in the actuating path without the need for an expensive hydraulic step-up mechanism. The gap arranged in the actuating path between the actuator and the valve closing body allows undisturbed thermal elongation of the actuator without the thermal elongation resulting in opening of the fuel injector.
The method according to the present invention for operating such a fuel injector having the features of Claim 7 has the advantage that, in order to open the fuel injector, the gap provided in the actuating path does not have to be overcome. Instead, the temperature-dependent elongation of the actuator is continuously measured before each actuating stroke of the actuator or in fixedly predefined time intervals. When the actuator is actuated, initially a first electrical actuating voltage is applied to it, which causes the actuator to expand so that the gap ideally disappears or is at least minimized. Subsequently a higher second electrical actuating voltage is applied to the actuator, which results in immediate opening of the fuel injector.
The measures given in the subclaims provide advantageous refinements of and improvements on the fuel injector presented in Claim 1 and the method of operating the fuel injector presented in Claim 7.
The gap is preferably arranged between an actuator flange connected to the actuator and a valve needle connected to the valve closing body. The gap is preferably filled with a gaseous medium, air in particular, which can be rapidly vented when the actuator is operated. The width of the gap is preferably dimensioned such that it is ensured that, when the actuator is in the non-energized rest state, the gap is not bridged due to temperature elongation of the actuator over the entire range of temperatures that may prevail during the operation of the fuel injector. This allows the fuel injector to be operated in a wide range of temperatures.
In an inward-opening fuel injector, the gap is preferably located on the side of the actuator that faces away from the valve closing body, while in an outward-opening fuel injector the gap is preferably located on the side of the actuator that faces the valve closing body.
The temperature-dependent elongation of the actuator can be measured, for example, by measuring the capacitance of the actuator. Since the actuator normally has a plurality of piezoelectric layers which are provided with electrodes, thermal expansion of the piezoelectric actuator results in an increase in the distance between the electrodes and therefore in a decrease in the capacitance. The temperature-dependent elongation of the actuator can then be calculated back from the measured capacitance. As an alternative, it may be sufficient to measure the temperature of the actuator if the coefficient of thermal expansion of the actuator is known with sufficient accuracy. Then the temperature-dependent elongation of the actuator at the measured temperature can then be calculated back from the actuator temperature measurement. Measuring the capacitance of the actuator and the temperature of the actuator can be combined to improve accuracy.